1. Field of the Invention
The present invention relates to a jib stretching and folding device in a wheel type crane such as a truck crane and a rough terrain crane for stretching and folding a jib in a so-called jib twisting operation wherein the jib is twisted.
2. Background of the Related Art
Such a jib stretching and folding device adopting the jib twisting, operation is described in Japanese Patent Publication No. 63-3827 and Japanese Utility Model Laid-open Publication No. 63-173185, for example. In this prior art, the jib is so folded as to be vertically disposed and laid aside a side surface of a boom. A jib stretching operation is carried out by the steps of suspending the jib in the above-mentioned folded position, then twisting the jib about a vertical axis by about 270.degree., then raising the jib, and finally stretching the jib from a forward end of the boom. In contrast, a jib folding operation is carried out by the steps of lowering the jib from the stretched position, then twisting the jib, and then drawing the jib toward the side surface of the boom.
However, in the prior art device as described in the above cited references, the jib twisting step in the stretching or folding operation is manually carried out. Accordingly, in case of a large jib to be used in a large crane, the twisting operation of the jib is troublesome. Furthermore, since such a manual operation for twisting the jib is necessary during the course of the jib stretching or folding operation, the stretching or folding operation cannot be made continuous.